The present invention relates to a dust-proofing device installed for the purpose of preventing the adhesion of foreign substances to a photomask used in a process of producing high density integrated circuits such as LSIs and VLSIs, and, more particularly, to a photomask equipped with a dust-proofing device which photomask is superior in the transmittance of ultraviolet rays in a short wavelength region and in light resistance and an exposure method using the photomask.
A pellicle having a structure in which as shown in FIG. 3, an organic thin film formed of nitrocellulose, a fluororesin or the like and having a thickness of several μm is applied to one side of a frame made of a metal or the like by using an adhesive or the like has been used as a dust-proofing device installed on photomasks used in the production of high density integrated circuits such as LSIs and VLSIs with the intention of preventing the adhesion of foreign substances such as dusts. The pellicle is bonded and secured to the photomask after the adhesive is applied to one side of the frame. The height of the frame is about 3 to 5 mm in general and the surface of the pellicle film is distant from the pattern surface of the photomask by the height of the frame. Therefore, if fine dusts are stuck to the surface of the pellicle film, the image of these dusts is neither formed on nor transferred to a semiconductor wafer to which a resist is applied during exposure.
However, in recent, there has been a strong demand for lithographic technologies enabling the formation of integrated circuits by using a finer line width with the intention of improving the integration of VLSIs and a progress in the use of exposure light source shaving shorter wavelengths has been made. For example, the trend of the light sources for lithography steppers is toward the use of ArF (193 nm) excimer lasers and F2 lasers (157 nm) from the use of a conventional g-line (436 nm), i-line (365 nm), and KrF excimer laser (248 nm).
When exposure wavelengths are shorter, particularly in the vacuum ultraviolet region less than 180 nm, this brings about the drawback that the light transmittance of organic materials such as nitrocellulose is insufficient. Also, because a ArF excimer laser and F2 laser have high energy, pellicles constituted of organic materials have the drawback that its film is denatured and a loss of the film is caused.
In order to obtain high light transmittance in a shorter wavelength region, there is a method using a glass pellicle formed of a synthetic quartz glass made into a thin film 1 mm in thickness as described in the publication of Japanese Patent Application Laid-Open No. 2001-83690. However, the usual atmosphere including oxygen are present inside of the quartz glass pellicle formed on a photomask, posing the problem that the oxygen absorbs light in the case of shorter wavelength light sources, especially, a 157 nm F2 laser, therefore, such devices that the atmosphere in the pellicle is replaced by inert gas, such as nitrogen, which does not absorb light, must be therefore taken. Also, the conventional methods have the problem that since the frame must be secured to the pellicle and the frame must be secured to the photomask by using an adhesive, repeated exposure causes degassing from the adhesive and the decomposition of the adhesive with the result that foreign substances are thereby stuck to the photomask pattern, which causes defects. The problems concerning the sticking of foreign substances caused by the adhesive is made more significant as the exposure wavelength is shorter.
As methods using no pellicle, there are descriptions concerning a photomask having a structure in which a thin plate glass is applied only to the effective parts of the a photomask by using a photocurable adhesive in the publication of Examined Japanese Patent Application No. S61-54211 and also concerning a structure obtained by applying an adhesive to all or part of a photomask and by applying a transparent substrate to the photomask by the adhesive in the publication of Japanese Patent Application Laid-Open No. S55-121443. However, if an adhesive is present in the exposed region of the photomask, repeated exposure causes degassing from the adhesive and the decomposition of the adhesive, giving rise to the problems such as the defects of a photomask caused by foreign substances and partial coloring and reduced ultraviolet transmittance caused by the denaturation of the adhesive. These problems caused by an adhesive are more significant as the exposure wavelength is shorter, giving rise to the drawback that these methods are not practically usable.